1. Field of the Invention
The present invention relates to a display unit. More particularly, this invention relates to a display unit showing an improved light extraction efficiency and realized by using diffractive elements.
2. Description of the Related Art
There has been remarkable progress in developing a spontaneous light emission type display unit using a fluorescent substance such as electron-emitting device type displays, organic EL displays and LED displays in recent years.
Electron-emitting device type displays are designed to use electrons emitted from an electron source as excitation source, excite a light-emitting layer typically made of a fluorescent substance to emit light and extract the emitted light to the outside. Organic EL displays and LED displays are designed to inject an electric current into a light-emitting layer as excitation source, make the light-emitting layer emit light and extract the emitted light to the outside. With any of the above arrangements, the light emitted from a light-emitting layer is extracted to the outside by way of a front panel or an electrode.
However, the light-emitting layer, the front panel and the electrode give rise to differences of refractive index at the interfaces thereof and, therefore, the part of the light emitted from the light-emitting layer and showing an emission angle not smaller than the critical angle is subjected to total reflection. Then, that part of the light cannot be extracted to the outside to consequently reduce the light excitation efficiency and lower the brightness of the display unit.
Methods of forming a diffraction grating at the front surface side or at the rear surface side of a light-emitting layer and diffracting the light emitted from the light-emitting layer by means of the diffraction grating in order to efficiently extract light to the outside have been developed as techniques of improving the light extraction efficiency. Japanese Patent Application Laid-Open No. 2004-349111 proposes a structure having an interlayer made of a thin film and arranged between a light-emitting layer and a diffraction grating to raise the light excitation efficiency and allow an electrode to be prepared on a stable basis and the light-emitting layer to emit light excellently.